First- and second-row turbine blades are used in very high temperatures and therefore are provided, for the purpose of protection from oxidation and corrosion as well as heat, with metallic and/or thermal layers.
Turbine blades in particular have, in addition, cooling air holes, out of which cooling air flows.
The boundary surfaces of layers, between adjacent layers and with the substrate, can cause crack initiation.
On their surface on the outermost layer, these cooling air holes have a diffuser in order that a protective layer of air flows over the surface of the turbine blade. Since the diffuser represents a widening of the cylindrical lower portion, a large amount of material must be removed.